A carbon nanotube is a graphite sheet curled in a cylindrical shape. According to the number of graphite sheets, carbon naotubes are classified into single-walled carbon nanotubes having a single graphite sheet, double-walled carbon nanotubes having double graphite sheets, and multiwalled carbon nanotubes having three or more graphite sheets.
Since the double-walled carbon nanotube has the advantages of both the single-walled carbon and the multiwalled carbon nanotube, its various applications in an electron emission device, an electronic device, a sensor, a high strength composite material and the like are anticipated. To use the double-walled carbon nanotube, it is essentially requested to synthesize a high purity double-walled carbon nanotube inexpensively and massively.
As methods of synthesizing the double-walled carbon nanotube, there are an electric discharge method and a vapor phase synthesis method. When using the electric discharge method, an amorphous carbon material is generated as a by-product in addition to carbon nanotubes. Accordingly, to obtain high purity carbon nanotubes, thermal and chemical filtration is essentially required. Also, it is difficult to massively and inexpensively synthesize carbon nanotubes using the electric discharge method.
Due to the above difficulties, the vapor phase synthesis method is gaining popularity as an alternative to synthesize the high purity carbon nanotube inexpensively and massively. Then, when double-walled carbon nanotubes are synthesized by various vapor phase synthesis methods reported up to now, production yield of the double-walled carbon nanotube is very low, and massive single-walled nanotube and amorphous carbon particles are created together with the double-walled carbon nanotube.